In order to implement the future wearable electronic apparatus, a transparent display, or the like, it is necessary to develop an electronic device that is bent, is extensible, and has excellent performance.
Two-dimensional materials such as graphene, molybdenum disulfide, and the like, have been considered as a material for fabricating the next generation electronic device, after silicon. Recently, black phosphorus evaluated as a material that may substitute for molybdenum disulfide having a band gap is an allotrope having the same element as that of phosphorus, but having a property different from that of phosphorus, and is a material having iron gray metallic luster and having an appearance similar to that of graphite.
Particularly, the graphene has an insufficient direct electron band gap between a valence band and a conduction band, such that it has a limitation in substituting for silicon, which is a typical semiconductor, but the black phosphorus has a direct band gap that may be tuned depending on a thickness thereof, such that it may be operated in a wide wavelength range from a visible ray to a near infrared ray.
The band gap is a unique physical quantity of a material, and when the band gap is close to 0, a current easily flows in the material, such that the material becomes a conductor, and when the band gap becomes large, a current does not easily flow in the material, such that the material becomes an insulator. Therefore, when a magnitude of the band gap may be freely tuned, it is possible to manipulate an electrical property of the material from a conductor to a non-conductor.
In order to lower an operating voltage of a transistor and decrease heat generated in the transistor, the transistor should be designed at a very thin thickness of an atomic level. Therefore, many studies on the black phosphorus, which is a material satisfying this demand, have been conducted.
However, the black phosphorus is not stable due to a high reaction speed in the air, such that a natural oxide film is generated. The natural oxide film hinders a flow of current amount on a surface or an interface of the black phosphorus with the passage of time, thereby deteriorating properties of an electronic device. Therefore, a method of effectively removing the oxide film is necessarily required in order to use a black phosphorus thin film in the electronic device.
Meanwhile, the black phosphorus having bulk band energy of about 0.3 eV has high photoluminescence properties through direct transition regardless of a thickness, and may adjust variable band energy from a near infrared to a visible region particularly at 3 nm or less. Currently, a black phosphorus thin film may be formed by a mechanical exfoliation method. However, in the mechanical exfoliation method, it is very difficult to fabricate black phosphorus ultrathin film due to formation of a natural oxide film.